The present invention relates to new and distinctive honeydew melons (Cucumis melo) having high percent soluble solids and improved firmness. The invention further relates to honeydew melon hybrids SSC 112, SSC 135 and SSC 118, and inbreds 10217-3, 10233-1, 11466, 10888, 10288-1 and 11876-1. All publications cited in this application are herein incorporated by reference.
There are numerous steps in the development of any new, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety or hybrid an improved combination of desirable traits from the parental germplasm. These important traits may include higher yield, field performance, fruit and agronomic quality such as sugar levels, small cavity size, flesh color or texture, rind thinness, resistance to diseases and insects, and tolerance to drought and heat.
Practically speaking, all cultivated forms of honeydew melon belong to the highly polymorphic species Cucumis melo L. that is grown for its edible fruit. As a crop, honeydew melons are gown commercially wherever environmental conditions permit the production of an economically viable yield. Honeydew melon production and consumption have been increasing in the United States since 1993. Leading producers of fresh market honeydew melons in the U.S. are California, Arizona and Texas (1997 figures). The total U.S. crop of honeydew melons was produced on 29,400 acres, with a total yield of 5,795,000 cwt (average yield=197 cwt/acre), and a total value of $109,394,000 in 1997.
The honeydew melon is one of the most popular melons, belonging to the Inodorus group of C. melo that also includes the casaba melon. Of course, the best known member of the winter melon group is honeydew melon. Honeydew is an American name for the French variety ‘White Antibes,’ which was grown for many years in southern France and Algeria for foreign shipment. While the honeydew melon plant is similar to the casaba melon, except for more lobing of the leaf, the fruits are distinctive. They are round to slightly oval, about 8 inches long, and are fairly smooth, depending on the variety, with no netting or ribs. Some soft hairs are present on the surface in early stages; these hairs disappear when the fruit is ripe. Rind color is greenish white when immature, becoming somewhat creamy yellow when ripe. The flesh is light green, thick, juicy, sweet, and uniquely flavored. Newer varieties include orange fleshed honeydews.
Cucumis melo is a member of family Cucurbitaceae. The Cucurbitaceae is a family of about 90 genera and 700 to 760 species. The family includes pumpkins, squashes, gourds, watermelon, loofah and several weeds. The genus Cucumis, to which honeydew belongs, includes about 70 species. Cucumis melo includes a wide range of cultivated plants. Although crosses outside the species are sterile, intraspecific crosses are generally fertile, resulting in a wide range of variation. The more common cultivated plants fall into four main groups. First are the true cantaloupes of Europe. These have thick, scaly, rough, often deeply grooved, but not netted rinds. Second are the muskmelons, mostly grown in the United States, where they are called cantaloupes. These have finely netted rinds with shallow ribs. Third are the casaba or winter melons with large fruits. These have smooth, often yellow rinds. The honeydew melons are in this third group. Fourth are a group of elongated melons of India, China and Japan which are grown as vegetables.
Honeydew melon is a simple diploid species with twelve pairs of highly differentiated chromosomes. Large field spaces are required for honeydew melon and there is a need for labor-intensive hand-pollination for selfing as well as cross pollination. Honeydew melon flowers open after sunrise; the exact time depends on environmental conditions such as sunlight, temperature and humidity. The flower closes permanently in the afternoon of the same day Almost all pollen is collected and transferred before noon. Typically flowers are staminate although some are also hermaphroditic. Although hermaphroditic flowers are self-fertile, they are incapable of performing self-pollination. Insects are required for pollination. The primary pollinators are bees, particularly honeybees.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar, pureline cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location will be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences choice of the breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars. Various recurrent selection techniques are used to improve quantitatively inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination and the number of hybrid offspring from each successful cross.
Each breeding program should include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goal and objectives, but should include gain from selection per year based on comparisons to an appropriate standard, overall value of the advanced breeding lines, and number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.). Promising advanced breeding lines are thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for three or more years. The best lines are candidates for new commercial cultivars; those still deficient in a few traits may be used as parents to produce new populations for further selection.
These processes, which lead to the final step of marketing and distribution usually take from eight to 12 years from the time the first cross is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
A most difficult task is the identification of individuals that are genetically superior, because for most traits the true genotypic value is masked by other confounding plant traits or environmental factors. One method of identifying a superior plant is to observe its performance relative to other experimental plants and to a widely grown standard cultivar. If a single observation is inconclusive, replicated observations provide a better estimate of its genetic worth.
The goal of honeydew melon plant breeding is to develop new, unique and superior honeydew melon cultivars and hybrids. The breeder initially selects and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. The breeder can theoretically generate billions of different genetic combinations via crossing, selfing and mutations. The breeder has no direct control at the cellular level. Therefore, two breeders will never develop the same line, or even very similar lines, having the same honeydew melon traits.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under unique and different geographical, climatic and soil conditions and further selections are then made during and at the end of the growing season. The cultivars that are developed are unpredictable because the breeder's selection occurs in unique environments with no control at the DNA level (using conventional breeding procedures), and with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. The same breeder cannot produce the same cultivar twice by using the same original parents and the same selection techniques. This unpredictability results in the expenditure of large amounts of research monies to develop superior new honeydew melon cultivars.
The development of new honeydew melon cultivars requires the development and selection of honeydew melon varieties, the crossing of these varieties and selection of superior hybrid crosses. The hybrid seed is produced by manual crosses between selected male-fertile parents or by using male sterility systems. These hybrids are selected for certain single gene traits such as pod color, flower color, pubescence color or herbicide resistance which indicate that the seed is truly a hybrid. Additional data on parental lines, as well as the phenotype of the hybrid, influence the breeder's decision whether to continue with the specific hybrid cross.
Pedigree breeding and recurrent selection breeding methods are used to develop cultivars from breeding populations. Breeding programs combine desirable traits from two or more cultivars or various broad-based sources into breeding pools from which cultivars are developed by selfing and selection of desired phenotypes. The new cultivars are evaluated to determine which have commercial potential.
Pedigree breeding is used commonly for the improvement of self-pollinating crops. Two parents that possess favorable, complementary traits are crossed to produce an F1. An F2 population is produced by selfing one or several F1s. Selection of the best individuals may begin in the F2 population; then, beginning in the F3, the best individuals in the best families are selected. Replicated testing of families can begin in the F4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F6 and F7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new cultivars.
Mass and recurrent selections can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals is either identified, or created, by intercrossing several different parents. The best plants are selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population in which further cycles of selection are continued.
Backcross breeding has been used to transfer genes for a simply inherited, highly heritable trait into a desirable homozygous cultivar or inbred line which is the recurrent parent. The source of the trait to be transferred is called the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent.
The single-seed descent procedure in the strict sense refers to planting a segregating population, harvesting a sample of one seed per plant, and using the one-seed sample to plant the next generation. When the population has been advanced from the F2 to the desired level of inbreeding, the plants from which lines are derived will each trace to different F2 individuals. The number of plants in a population declines each generation due to failure of some seeds to germinate or some plants to produce at least one seed. As a result, not all of the F2 plants originally sampled in the population will be represented by a progeny when generation advance is completed.
In a multiple-seed procedure, honeydew melon breeders commonly harvest one or more pods from each plant in a population and thresh them together to form a bulk. Part of the bulk is used to plant the next generation and part is put in reserve. The procedure has been referred to as modified single-seed descent or the pod-bulk technique.
The multiple-seed procedure has been used to save labor at harvest. It is considerably faster to thresh pods with a machine than to remove one seed from each by hand for the single-seed procedure. The multiple-seed procedure also makes it possible to plant the same number of seeds of a population each generation of inbreeding. Enough seeds are harvested to make up for those plants that did not germinate or produce seed.
In addition to phenotypic observations, the genotype of a plant can also be examined. There are many laboratory-based techniques available for the analysis, comparison and characterization of plant genotype; among these are Isozyme Electrophoresis, Restriction Fragment Length Polymorphisms (RFLPs), Randomly Amplified Polymorphic DNAs (RAPDs), Arbitrarily Primed Polymerase Chain Reaction (AP-PCR), DNA Amplification Fingerprinting (DAF), Sequence Characterized Amplified Regions (SCARs), Amplified Fragment Length polymorphisms (AFLPs), Simple Sequence Repeats (SSRs—which are also referred to as Microsatellites), and Single Nucleotide Polymorphisms (SNPs).
Isozyme Electrophoresis and RFLPs have been widely used to determine genetic composition. Shoemaker and Olsen, (Molecular Linkage Map of Honeydew melon (Glycine max L. Merr.) p 6.131-6.138 in S. J. O'Brien (ed) Genetic Maps: Locus Maps of Complex Genomes, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., (1993)) developed a molecular genetic linkage map that consisted of 25 linkage groups with about 365 RFLP, 11 RAPD and three classical markers and four isozyme loci. See also, Shoemaker, R. C., RFLP Map of Honeydew melon, p 299-309, in Phillips, R. L. and Vasil, I. K. (eds.) DNA-Based Markers in Plants, Kluwer Academic Press, Dordrecht, the Netherlands (1994).
SSR technology is currently the most efficient and practical marker technology; more marker loci can be routinely used and more alleles per marker locus can be found using SSRs in comparison to RFLPs. For example, Diwan and Cregan described a highly polymorphic microsatellite locus in honeydew melon with as many as 26 alleles. (Diwan, N. and Cregan, P. B., Theor. Appl. Genet. 95:22-225, 1997.) SNPs may also be used to identify the unique genetic composition of the invention and progeny varieties retaining that unique genetic composition. Various molecular marker techniques may be used in combination to enhance overall resolution.
Molecular markers, which includes markers identified through the use of techniques such as Isozyme Electrophoresis, RFLPs, RAPDs, AP-PCR, DAF, SCARs, AFLPs, SSRs, and SNPs, may be used in plant breeding. One use of molecular markers is Quantitative Trait Loci (QTL) mapping. QTL mapping is the use of markers which are known to be closely linked to alleles that have measurable effects on a quantitative trait. Selection in the breeding process is based upon the accumulation of markers linked to the positive effecting alleles and/or the elimination of the markers linked to the negative effecting alleles from the plant's genome.
Molecular markers can also be used during the breeding process for the selection of qualitative traits. For example, markers closely linked to alleles or markers containing sequences within the actual alleles of interest can be used to select plants that contain the alleles of interest during a backcrossing breeding program. The first genetic maps of melon made using molecular markers were published in 1996 (Baudracco et al., Theor. Appl. Genet. 93:57-64) and in 1997 (Wang et al., Theor. Appl. Genet. 95:791-797). At the time few horticultural traits had been placed for melon, but since then, Perin et al. (Cucurbitaceae 1998:370-376) have constructed a genetic map of melon with molecular markers (AFLP and SSR) and horticultural traits, including Fusarium wilt resistance (gene Fom-1 and Fom-2) Aphis gssypii resistance (gene Vat), melon necrotic spot virus resistance (gene nvs), five carpels (gene p), green flesh color (gene gf) and various fruit characters. These markers may be advantageously used for breeding through Marker Assisted Selection as described in Zheng et al., Theor. Appl. Genet. 99:453-463, where PCR-based CAPS (cleaved amplified polymorphic sequences) were used as markers linked to resistance/susceptibility for Fusarium wilt in melon. The markers can also be used to select toward the genome of the recurrent parent and against the markers of the donor parent. Using this procedure can attempt to minimize the amount of genome from the donor parent that remains in the selected plants. It can also be used to reduce the number of crosses back to the recurrent parent needed in a backcrossing program. The use of molecular markers in the selection process is often called Genetic Marker Enhanced Selection. Molecular markers may also be used to identify and exclude certain sources of germplasm as parental varieties or ancestors of a plant by providing a means of tracking genetic profiles through crosses as discussed more fully hereinafter.
Mutation breeding is another method of introducing new traits into honeydew melon varieties. Mutations that occur spontaneously or are artificially induced can be useful sources of variability for a plant breeder. The goal of artificial mutagenesis is to increase the rate of mutation for a desired characteristic. Mutation rates can be increased by many different means including temperature, long-term seed storage, tissue culture conditions, radiation (such as X-rays, Gamma rays, neutrons, Beta radiation, or ultraviolet radiation), chemical mutagens (such as base analogues like 5-bromo-uracil), antibiotics, alkylating agents (such as sulfur mustards, nitrogen mustards, epoxides, ethyleneamines, sulfates, sulfonates, sulfones, or lactones), azide, hydroxylamine, nitrous acid or acridines. Once a desired trait is observed through mutagenesis the trait may then be incorporated into existing germplasm by traditional breeding techniques. Details of mutation breeding can be found in “Principles of Cultivar Development” by Fehr, Macmillan Publishing Company, 1993.
The production of double haploids can also be used for the development of homozygous varieties in a breeding program. Double haploids are produced by the doubling of a set of chromosomes from a heterozygous plant to produce a completely homozygous individual. For example, see Wan et al., Theor. Appl. Genet., 77:889-892, 1989.
Descriptions of other breeding methods that are commonly used for different traits and crops can be found in one of several reference books (e.g., Allard, 1960; Simmonds, 1979; Sneep et al., 1979; Fehr, 1987).
Proper testing should detect any major faults and establish the level of superiority or improvement over current cultivars. In addition to showing superior performance, there must be a demand for a new cultivar that is compatible with industry standards or which creates a new market. The introduction of a new cultivar will incur additional costs to the seed producer, the grower, processor and consumer for special advertising and marketing, altered seed and commercial production practices, and new product utilization. The testing preceding release of a new cultivar should take into consideration research and development costs as well as technical superiority of the final cultivar. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.
Honeydew melon, Cucumis melo, is an important and valuable field crop. Thus, a continuing goal of honeydew melon plant breeders is to develop stable, high yielding honeydew melon cultivars that are agronomically sound. The reasons for this goal are obviously to maximize the amount of fruit produced on the land used as well as to improve the fruit agronomic qualities. To accomplish this goal, the honeydew melon breeder must select and develop honeydew melon plants that have traits that result in superior cultivars.